1. Field of the Invention
The present invention relates to an electrophotographic machine, and, more particularly, to determining a position of a laser beam in an electrophotographic machine, such as a laser printer.
2. Description of the Related Art
In an in-line color laser image printing process, the print medium typically passes through four color developing stations in series, with the colors being black, magenta, cyan and yellow. In order for the multi-color laser printer to print at the same speed as a monochrome laser printer, photoconductive drum exposures must occur for all four colors simultaneously. Thus, alignment of the four color developing stations in both the process direction (feed direction of the print medium) and scan direction (across the page) is critical.
The process location of each scanning laser beam must overlap to prevent color mis-registration in the process direction. Each color must have an adjustment to correct for process direction misalignment because each color has a scanning laser beam following a separate optical path. Although the laser beams can be aligned when the laser printer is first assembled, thermal changes occurring during operation of the laser printer can cause subsequent misalignment of the laser beams.
It is known to use a horizontal synchronization (HSYNC) sensor to determine the location of the start of the scan of the laser beam across the photoconductive drum. The HSYNC sensor has a rectangular photosensitive surface which is placed somewhere in the laser print head near the start of a scan line. When the laser beam strikes the HSYNC sensor surface at the start of a scan line, the photodiode sensor detects the presence of the laser beam and thereby identifies the location of the laser beam. Such rectangular sensors, however, cannot be used to determine a location of the laser beam in the process direction.
What is needed in the art is a low-cost method of aligning multiple laser beams in a process direction of a laser printer such that all of the laser beams can operate simultaneously to thereby achieve the same printing speed as that of a monochrome laser printer.
The present invention provides a method of real time detection of process direction location of multiple scan lines of a multicolor electrophotographic machine, such as a laser printer.
The invention comprises, in one form thereof, a method of determining a position of a laser beam in an electrophotographic machine. A sensor device having a laser beam receiving surface with a first edge and a second edge is provided. The second edge is nonparallel to the first edge. The laser beam is scanned across the receiving surface in a scan direction perpendicular to a process direction. The laser beam intersects each of the first edge and the second edge of the receiving surface during the scanning. A time period between when the laser beam intersects the first edge and when the laser beam intersects the second edge of the receiving surface is measured. A process position of the laser beam along the process direction is calculated based upon the measured time period.
An advantage of the present invention is that the laser beams of a multicolor electrophotographic machine can be aligned in the process direction in real time while the machine is operating.
Another advantage is that only one sensor is required to detect a laser beam location in both the scan and process directions.